For example, a construction machine such as a hydraulic excavator can perform excavation by a working mechanism (front) composed of a boom, an arm, a bucket and the like, travel of a machine by a lower traveling structure, revolution of an upper revolving structure and the like. Therefore, the hydraulic excavator is provided with an operating lever that is operated by an operator to perform the excavation, the travel, the revolution and the like, a plurality of hydraulic actuators for performing these movements of the excavation, the travel, the revolution and the like, a main pump that delivers pressurized oil for driving each of the hydraulic actuators, an engine that drives the main pump, a plurality of control valves that distribute the pressurized oil to each of the hydraulic actuators in response to the lever operating of an operator, and a pilot pump that is driven by the engine to generate a pilot pressure for controlling opening/closing of the control valve. This construction machine controls the pilot pressure in accordance with the operating amount of the operating lever to distribute the pressurized oil to each of the hydraulic actuators in response to the lever operation by an operator, thus enabling the machine to move according to an intent of the operator.
Here, general hydraulic excavators control the pilot pressure by a hydraulic circuit. In this case, some of the hydraulic excavators are designed such that control by a controller is added to the control of the pilot pressure to prevent the hydraulic excavator from excavating excessively over a preset target excavating surface or the bucket from colliding with a vehicle body including a cab of the hydraulic excavator. This type of hydraulic excavator is provided with a posture sensor (for example, an tilt angle sensor, a potentiometer or the like) that measures a posture of the vehicle body or the working mechanism, a pressure sensor that measures a pilot pressure in accordance with an operating amount of the operating lever, a proportional solenoid valve that reduces the pilot pressure generated in accordance with the lever operating amount, another proportional solenoid valve that increases the pilot pressure regardless of the lever operation, and a controller that drives the proportional solenoid valve based upon posture information of the vehicle body or the working mechanism by the posture sensor and lever operating information by the pressure sensor. In this case, the controller corrects the movement of the working mechanism by reducing or increasing the pilot pressure in such a manner as to prevent the working mechanism from deviating from a predetermined spacious area when an operator operates the working mechanism.
On the other hand, there are some hydraulic excavators in which an electrical lever is adopted as the operating lever, and the pilot pressure is controlled only by the controller without providing a hydraulic circuit for controlling the pilot pressure. This hydraulic excavator is provided with the electrical lever that outputs an electrical operating signal in accordance with a lever operating amount, a proportional solenoid valve that controls pilot pressures of a plurality of hydraulic actuators, and a controller that drives the proportional solenoid valve based upon an operating signal that is outputted by the electrical lever. In this case, the controller controls each of the hydraulic pilot pressures in accordance with the lever operating amount to operate the machine. Further, there are other hydraulic excavators that are provided with a posture sensor that measures a posture of the vehicle body or the working mechanism. In this case, the controller controls the pilot pressure of each of the hydraulic actuators such that the working mechanism does not deviate from a predetermined spacious area, making it possible to operate the working mechanism.
These hydraulic excavators have a possibility that in a case where some malfunction occurs or noises are mixed in the controller, the controller drives the proportional solenoid valve in error. In this case, even when the operating lever is returned back to a neutral position, the machine does not stop possibly. In contrast thereto, for example, Patent Document 1 discloses a drive control device of a hydraulic machine that is provided with an electrical lever that outputs a lever operating amount signal in accordance with an operating amount, a neutral position signal outputting section configured to output a neutral position signal when the electrical lever is in a neutral position, a controller that drives a proportional solenoid valve that controls a pilot pressure of each of the actuators, based upon the lever operating amount signal, and a blockade device that performs an on/off operation of a drive signal between the controller and the proportional solenoid valve based upon the neutral position signal. The blockade device blocks the drive signal of the proportional solenoid valve of the concerned actuator when the operating lever of each of the actuators is in the neutral position. Accordingly, even when abnormality of the controller occurs, it is possible to stop the machine by returning the operating lever to the neutral position.